In the infrared (IR) region of the electromagnetic spectrum many inorganic and almost all organic chemicals have spectra that can be used to identify them and measure their concentration in mixtures. Unlike other analytical techniques such as mass spectrometry, chromatography and wet chemistry, IR analysis is nondestructive and does not disturb the sample.
A considerable amount of quantitative and semiquantitative analysis is conducted using Fourier Transform IR (FTIR) and wavelength dispersive IR spectrometers. This work uses a variety of inconvenient and difficult to use micro cells and pumped flow cells.
The multiple internal reflection (MIR) technique has been developed and refined in order to simplify spectrographic analysis. This technique overcomes the need for very short path length transmission cells by making use of the fact that when IR radiation propagating within a transparent medium is reflected internally from a surface, a portion of the radiation extends into the surrounding medium. This distance is typically in the 1-50 .mu.m range. As a result the intensity of the reflected light is reduced at those wavelengths at which the surrounding medium absorbs. The known art for analyzing strongly absorbing or scattering liquids uses flat plate MIR cells, very short path length transmission cells, rod crystals using refractive lenses, rod crystals using funnel shaped mirrors and cylindrical internal reflection (CIR) rods.
A known CIR device 10 shown in FIG. 1 is composed of an infrared transmitting material such as attenuated total reflection (ATR) crystal 12 which transmits light in the desired frequency range and has the geometry of a cylinder which is polished and pointed at both ends 14, 16. Usually IR radiation 18 is directed into one end 14 of the crystal 12 at a 90.degree. angle to the pointed surface. The radiation propagates through the CIR crystal 12 and makes reflections with its polished walls. 20. At each point of reflection shown by the arrows 22 an evanescent wave is formed which penetrates into the surrounding media 24 resulting in an interaction of the light wave with the medium outside of the crystal 12. After several other reflections of the light on the crystal walls 20, it passes out of the crystal at the other end 16 and is directed toward a detector 26 where the signal is analyzed. Comparison of a spectrum taken with no medium other than air to one containing inorganic or organic compounds surrounding the crystal permits the acquisition of an infrared, visible, ultraviolet or other spectrum. The CIR device 10 shown in FIG. 3 is described in Sting, U.S. Pat. No. 4,595,833 and by Wilks and Rein (P. A. Wilks Ind. Res. & Dev., 132, Sept. 1982 and by A. J. Rein and P. A. Wilks, Am. Lab. 14 (10), 153 (1982)).
Another known configuration for the crystal 30 shown in FIG. 2 is described in Harrick, U.S. Pat. No. 3,393,603. In this arrangement the crystal 30 has a pointed input/output end 32 and a flat reflector end 34 which returns the light 36.
The CIR technique may also be used for high pressure, stirred chemical reactions, as described by Moser et al. (W. R. Moser, J. E. Cnossen, A. W. Wang, and S. A. Krouse 187th National Am. Chem. Soc. Meet. April, 1984, St. Louis, MO. USA; ibid; J. Catalysis, 95, 21-23 (1985)). Known CIR-Reactors 40, such as illustrated in FIG. 3, permit limited unsatisfactory in situ reaction monitoring capabilities for a variety of catalytic reactions including heterogeneous gaseous solid reactions at high temperature and pressure. The ATR element 42 is located in high pressure vessel 44 as shown. The incident beam 46 enters the crystal 42 at one end, is attenuated and exits at the other end. The crystal is secured at the vessel 44 by means of step pressure seals 48. The stirrer 50 is provided to facilitate the reaction. It has also been proposed that CIR-Reactors may be used to provide reaction monitoring of zeolite synthesis, sol-gel inorganic oxide synthesis, and have the potential for monitoring biological systems which are usually studied in aqueous media.
The application of spectroscopy to chemistry and process control using NIR and IR spectrophotometers, FTIR, other spectrometers plus single point and multiple frequency measurement systems has been mainly restricted to the laboratory environment due to the lack of availability of convenient mechanisms for remotely locating measurement cells from instrument housings. There are several examples of locally mounted attachments for UV, visible and IR spectrometers that are commercially available and described in the literature and patents (see for example Wilks Jr., U.S. Pat. No. 3,370,502 and the Harrick and Sting Patents noted above). These devices all mount into or adjacent to the sample compartment of spectrometers using mirrors and/or transmissive optics for delivering and returning the light beam from the sample cell to the instrument detector. Although the stated purpose of some of these attachments is to provide for in situ monitoring of solid, liquid or gaseous samples including chemical reactions, these samples must generally be removed from their natural environment and conveyed to the instrument for measurement. This is often inconvenient since the measurement system is usually located in a clean laboratory environment remote from the location where the sample is produced. The validity of in vitro laboratory measurement versus actual in situ environment monitoring is subject to question. Laboratory methods are typically restricted in volume and environmental conditions. The timing between sampling a process line and measuring sample properties in the laboratory may result in sample degradation and provides no information regarding the actual process product during the time between samples. Furthermore, laboratory methods can place a valuable instrument in jeopardy of contamination and/or destruction depending upon the biological, chemical or physical nature of the sample and test environment.
The various optical systems which employ a CIR rod require complex and bulky optics for assuring that the light enters the crystal at an appropriate angle with respect to the central or long axis 38 of the crystal 12, 30. This is cumbersome, inconvenient and expensive to implement. In addition, it limits the placement of spectrographic equipment to closely proximate, line of sight arrangements with the reactor. Such restrictions make it difficult if not impossible to perform in situ measurements in a hostile environment.
The prior art involves the use of reflective and transmissive optics mounted locally to a spectrometer lacking the ability to locate the sample measurement remotely from the instrument. Alternatively, the prior art employs hollow reflective tubes for conductance of spectrometer radiation to an optical condenser and remote transmission or IRE cell. See for example W. M. Doyle & N. A. Jennings Spectroscopy 5 (1) 34-38 (1990)). The hollow rigid tube waveguide approach is inflexible and has demonstrated very poor optical efficiency and limited linear performance. It is a crude design but until the advent of the present invention, it has been the best available recourse for remote testing and immersion probe testing.